By Phil Mercer
BBC News, Sydney
Australian scientists have reported the discovery of coral reefs stretching 100km (62.5 miles) in the Gulf of Carpentaria, off the north coast.
They were located near Mornington Island, where deep murky water obscured them from the view of satellites.
Researchers have called it a major discovery that showed how little was known about Australia's continental shelf.
The newly discovered reefs could be at least 100,000 years old.
They were found inadvertently by scientists investigating a number of small reefs discovered on a previous expedition two years ago.
Sonar mapping equipment has produced an accurate picture of what was previously hidden from the gaze of satellites.
Researchers have said the discovery makes the Gulf of Carpentaria one of Australia's most important coral zones.
There could be as many as 50 small reefs, as well as this new long platform of coral that stretches for 100km to the west of Mornington Island.
Scientists are confident that more large reefs could be hiding in the deep tropical water. Environmentalists have said that these sorts of reefs are among the most bio-diverse ecosystems on earth.
They are urging the authorities to include them in a maritime national park being planned for the area.
The renowned Great Barrier Reef off north-eastern Australia is home to 1,500 species of fish and is considered to be the world's largest living structure.